Warning: Spoilers for Pokemon Omega Ruby and Alpha Sapphire ahead.
Pokemon X & Y introduced Mega Evolution to the series, a new feature which allowed players to transform the likes of Lucario, Mewtwo and Charizard into much more powerful versions of themselves. However, completing last year’s Pokemon Omega Ruby or Alpha Sapphire, reworkings of the classic Game Boy Advance titles, made the ‘Delta Episode’ available to players, end-game content which saw Mega Evolutions taking center stage once again.
This led many Pokemon fans to ask the question: why do Mega Evolutions appear in some games but not others? YouTube channel The Game Theorists actually have a very reasonable (and quite mind-blowing) explanation for this, arguing the point that each game actually exists inside its own dimension as part of a wider multiverse.
The multiverse theory posits that for each decision made in one universe, another is created to reflect what would have happened if a different decision would have been made. For instance, the tale of AZ in Pokemon X & Y details how the mysterious man created the “ultimate weapon,” which he built in order to revive his beloved Pokemon Floette. However, the ultimate weapon worked by sucking the life force out of other Pokemon, which led to the destruction of hundreds of species of the creatures. This is the universe in which Mega Evolution exists, and ascribing to the multiverse theory explains why Mega Evolution does not appear in any of the other games in the series.
However, the theory goes even deeper than that, explaining that each version of each iteration in the series also takes place in a different universe, explaining why different Pokemon can be found on different routes in Red and Blue, Gold and Silver and so on. Then it goes even deeper, claiming that each different cartridge of the game owned by each player is a separate universe in and of itself, meaning that trading/battling with friends is actually you opening up a rift in your Pokemon universe to another dimension, something which is actually referenced in one of the games.
If it’s all too confusing, then The Game Theorists go into greater detail in the below video. Prepare to have your knowledge of the Pokemon world thrown into doubt: